Flexible joints for tension piping systems are used extensively in aircraft duct systems, especially in compressor bleed air and anti-icing lines. While several different types of flexible joints are known, they have several disadvantages.
First, typical ball and socket joints, while providing flexibility, transmit the axial tension load due to pressure through a seat having a relatively large radius of curvature. Since the movement of resistance to angulation is proportional to the product of the axial force, the coefficient of friction and the radius of curvature of the bearing surface, the large seat radius results in a stiff joint. Stiff joints are undesirable since, while permitting angular rotation to accomodate flexure of the ducts, they impose high bending moments on the ducting and end connections, which may result in unacceptable stresses on engine ports, heat exchanges, airframe structures and the ducts themselves.
Flexible joints are also known which are formed via a gimbal having either an internal gimbal ring or an external gimbal ring. If the gimbal ring is located internally, the ring is subjected to large deflections, thereby requiring a heavier ring. In external gimbal ring joints, the ring does not cover and thus protect the internal seal coupling the pipes and does not provide a secondary seal limiting leakage if the primary seal fails.
Examples of these prior art joints are disclosed in the following U.S. Pat. Nos. 656,667 to Schmid; 2,613,087 to Alford; 3,219,365 to Webb; and 4,165,107 to Affa et al. In addition, such a prior art joint is disclosed in U.K. Pat. No. 2,107,816 to Ward.